--- title: "Basic requirements of a protection scheme" subject: "Switchgear and Protection" chapter: 1 key: "semester_6/switchgear_and_protection:basic-requirements-protection-scheme" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/basic-requirements-protection-scheme site: BCT Engineering --- # Basic requirements of a protection scheme Switchgear and Protection · Chapter 1: Principle of Power System Protection - Asked in: 2070 Magh, 4 marks; 2072 Magh, 4 marks; 2073 Bhadra, 4 marks (asked 3 times) - Video: [Essential Qualities of Protective Relay](https://www.youtube.com/watch?v=EQFHskT_9oo) — Electrical Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/basic-requirements-protection-scheme - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-1 ## Question What are the basic requirements of any protection scheme in a power system? Explain them with the help of suitable example. ## Short answer A protection scheme must be selective, fast, sensitive, reliable, stable, simple, and economical. For example, a restricted earth-fault relay set at 10–20% of rated current ensures sensitivity to high-resistance winding faults, while selectivity ensures only the nearest breaker trips. ## Answer A protection scheme must detect every fault quickly and remove only the faulty part, while staying stable during normal operation. The basic requirements (qualities) are: 1. **Selectivity (discrimination):** only the circuit breakers nearest to the fault should trip, so that the healthy part keeps supplying load. *Example:* for a fault on one outgoing 11 kV feeder of a substation, only that feeder breaker should open, not the incoming transformer breaker. 2. **Speed:** the fault must be cleared as fast as possible to limit damage to equipment, reduce burning at the fault point and keep generators in synchronism (transient stability). *Example:* a 132 kV line fault is cleared in about 3–5 cycles (relay ≈ 1 cycle + breaker 2–3 cycles). 3. **Sensitivity:** the relay must operate for the smallest fault current expected (e.g. a high-resistance earth fault at minimum generation). *Example:* a restricted earth-fault relay on a transformer is set at 10–20 % of rated current so it can see winding faults near the neutral. 4. **Reliability:** the scheme must operate correctly every time it is needed (dependability) and must not operate when not needed (security). Good design, quality CTs/relays, DC supply and regular testing give reliability. 5. **Stability:** the scheme must remain inoperative for faults outside its zone (through faults), for load swings and for magnetising inrush. *Example:* a transformer differential relay must not trip for an external fault on the LV feeder or for inrush current at switching-on. 6. **Simplicity and economy:** the scheme should be simple to maintain and its cost should be justified by the value of the protected equipment. A small distribution transformer is protected by fuses, while a large generator gets differential, earth fault, loss-of-excitation and other relays. 7. **Adequateness:** it should cover all the likely faults and abnormal conditions (overload, unbalance, over-voltage) of the equipment. These requirements partly conflict (more speed and sensitivity can reduce stability), so the designer reaches a balance suitable for each element. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. For a fault on one outgoing 11 kV feeder, which breaker should trip to show selectivity? A. Only that feeder's breaker B. The incoming transformer breaker only C. All feeder breakers and the incoming breaker D. The generator breaker at the source Answer: A. Selectivity means only the breaker nearest the fault trips so healthy parts keep supplying load. (Key point: Selectivity) 2. Which pair correctly describes the two parts of reliability? A. Speed (fast operation) and sensitivity (low pick-up) B. Selectivity and simplicity C. Dependability (operate when needed) and security (not operate when not needed) D. Dependability (never operate) and security (always operate) Answer: C. Reliability = dependable operation when required plus security against unwanted operation. (Key point: Reliability) 3. A transformer differential relay must NOT trip for which of these? A. A winding fault inside the transformer B. An external fault on the LV feeder or magnetising inrush C. A phase-to-earth fault near the neutral in the winding D. A fault inside the differential zone Answer: B. Stability means remaining inoperative for through-faults, load swings and inrush. (Key point: Stability) 4. Why must the designer balance the requirements? A. Greater simplicity always increases speed B. Higher reliability always reduces cost C. Selectivity and economy are identical D. Greater speed and sensitivity can reduce stability Answer: D. The requirements partly conflict, e.g. faster, more sensitive relays may operate wrongly for external events. 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